This application relates to methods of joining ceramic components in their green state. In particular, this application relates a method of thermally joining ceramic arc tube parts to form a unitary arc tube body for high intensity discharge (HID) lamps.
In general, commercial ceramic arc tubes used in high intensity discharge (HID) lamps are comprised of a polycrystalline alumina ceramic which may contain one or more additives to control grain growth. As a first step, alumina powder is mixed with a binder material such as a wax or thermoplastic and then formed into the desired shape by isostatic pressing, extrusion, or injection molding. The binder materials help the molded alumina piece retain its shape while the piece is in its xe2x80x9cgreen state,xe2x80x9d i.e., prior to binder removal and sintering. The binder is later removed the pieces are fired.
Since the arc tubes are fabricated from two or more pieces, it is necessary to form hermetic seals at the interfaces between the pieces which are capable of withstanding the high stresses, temperatures and corrosive chemicals present in an operating arc tube. The conventional method of assembling ceramic arc tube pieces involves several assembly and pre-sintering steps in which the pieces are aligned and sealed together by means of interference fits. The interference fits result from the differential shrinkage of the pieces during firing. In each of the assembly and pre-sintering steps, there exists an opportunity for misalignment or other errors to occur. Minimizing the number of firing cycles can improve the efficiency of the arc tube production process. Furthermore, the practice of using interference fits to form the hermetic seals requires high degree of control over dimensional tolerances and the shrinkage of the ceramic pieces during firing.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a method of assembling a ceramic body such as an arc tube which minimizes the number of firing and assembly steps.
It is further object of the invention to provide a method for forming hermetic seals in ceramic arc tubes which doesn""t rely on interference fits.
In accordance with one object the invention, there is provided a method of assembling a ceramic body, the ceramic body containing a binder material and comprising at least a first section having a first joining surface and a second section having a second joining surface, the method comprising the steps of:
(a) simultaneously heating the first and second joining surfaces to cause localized melting of the binder material;
(b) initially contacting the first joining surface with the second joining surface to form an interface region; and
(c) alternatively applying compression and stretching to the interface region to join the first section to the second section.
In accordance with another object of the invention, there is provided a method of assembling a ceramic arc tube in a green state comprising the steps of:
(a) securing a first axially symmetric arc tube section in a first holder, the first arc tube section having a first electrode-receiving member and a first cavity-forming member, the first cavity-forming member having a first annular joining surface;
(b) securing a second axially symmetric arc tube section in a second holder, the second arc tube section having a second electrode-receiving member and a second cavity-forming member, the second cavity-forming member having a second annular joining surface, the first and second arc tube sections containing a binder material;
(c) simultaneously heating the first and second annular joining surfaces to cause a localized melting of the binder material;
(d) initially contacting the first annular joining surface with the second annular joining surface to form an interface region; and
(e) joining the first section to the second section by initially displacing at least one of the sections in a forward direction and then displacing at least one of the sections in a reverse direction.